1. Field of the Invention
The present invention relates to a continuous type liquid ejection head that ejects a liquid, such as an ink, and a liquid ejection device in which the liquid ejection head is mounted. In the following description, an ink is taken as a typical example of the liquid.
2. Description of the Related Art
A variety of proposals have been made for liquid ejection heads mounted in liquid ejection devices represented by inkjet printing apparatus. Among them is a continuous type liquid ejection head. This type of liquid ejection head periodically vibrates ink at around 100 kHz by a vibration unit as the ink pressurized by a pump is ejected from an ejection opening in the form of an ink column just outside the ejection opening. The vibrations applied from the vibration unit form regular constrictions in the ink column according to the frequency given to the vibration unit, with the result that the constrictions in the ink column grow by the surface tension of the ink until the liquid column breaks up into a series of ink droplets.
In the continuous type liquid ejection head, to form a series of ink droplets, it is necessary to distinguish, according to print data, ink droplets used for printing from those not used. One such example involves selectively charging ink droplets with static electricity to deflect them by an electric field. A method called binary scheme uses uncharged ink droplets for printing and arrests and recovers charged ink droplets by gutters. To realize these functions, a charging electrode, a deflection electrode and a gutter are provided along ink flying paths from the ejection openings.
In recent years, to mitigate deformations of a print medium (curling and cockling) caused by water in ink ejected from the liquid ejection head, the use of highly viscous ink with a reduced amount of water is being studied. Some of the existing commercially available printers deal with the problem of print medium deformations due to water contained in ink by installing a drying unit of heater in the printer body. If the ejection of highly viscous ink becomes possible, this eliminates the need for the drying unit, reducing power consumption and the size of the printer. Further, the highly viscous ink ejection is also being considered for applications where the liquid ejection head device is used as a liquid application device and a pattern forming device to form patterns on functional materials of industrial products.
When a conventional continuous type liquid ejection head is used to eject highly viscous ink and break it up into a series of droplets at a frequency of around 100 kHz, a distance required to break up an ink column into droplets (hereinafter called a droplet forming distance) is more than a few millimeters, which is greater than that of low viscous ink. This is because the flow speed of the highly viscous ink is more retarded by the surface tension than that of a low viscous ink. The longer droplet forming distance inevitably leads to such problems as a degradation in landing precision and an increased size of the head.
To transform a highly viscous ink column into a series of droplets within a droplet forming distance almost equal to or shorter than that of the conventional continuous type liquid ejection head that uses a low viscous ink, an ink column, as it comes out of the ejection opening, needs to be reliably formed with constrictions. To this end, the head must be constructed to ensure that periodic vibrations (pressure variations) produced by the vibration unit can be transmitted efficiently to the ink column that is formed as the ink is ejected from the ejection opening. That is, the head needs to be constructed in a way that can impart with higher efficiency the force produced by the vibration unit to the ink being ejected from the ejection opening.
A method of ejecting a highly viscous ink has been known, as disclosed in Japanese Patent Laid-Open No. 2005-205752. In this method, an elastic member is formed in a pressure chamber that communicates with ejection openings from which ink is ejected. The elastic member is depressed and displaced by a piezoelectric member, which is an actuator, through a vibrating plate to reduce a volume of the pressure chamber. In this construction, since the vibrating plate is pressed against the elastic member, the deflection of the vibrating plate can be minimized. As a result, the major change in the volume of the pressure chamber is caused not by the deflection of the vibrating plate but by the compressive deformation of the elastic member. Therefore, that portion of the force of the actuator which is used in deflecting the vibrating plate becomes minimal, allowing the force that the actuator has applied to the vibrating plate to be transmitted to ink more efficiently.
However, in the construction of Japanese Patent Laid-Open No. 2005-205752, since the elastic member and the ejection openings are remote from each other, there is a pressure loss, which in turn attenuates the force that the actuator has applied to the vibrating plate before it reaches the ejection openings. Even if a large pressure change is produced near the vibrating plate, it is attenuated to a small pressure change near the ejection openings. When the construction of Japanese Patent Laid-Open No. 2005-205752 is applied to a continuous type liquid ejection head, it is considered not possible to give large enough flow changes to an ink column being ejected. So, this construction is not an efficient one to realize the ejection of ink with even higher viscosity.
Further, in the conventional continuous type liquid ejection head that uses low viscous ink (5 cP or lower), the ink column ejected from the ejection opening is formed with regular constructions by vibrations of the vibrating unit and then separated into a series of droplets by the surface tension of the ink. Here the distance required to separate an ink column into a series of droplets is 1 mm or less. However, where a high viscosity ink is used, the flow speed of the ink becomes slower, so that to separate the ink column into a series of droplets by the surface tension alone requires a distance of a few millimeters or more. The longer droplet forming distance gives rise to problems such as degradations of landing precision and an increased size of the print head. To keep the droplet forming distance for high viscosity ink equal to or less than that of the conventional continuous type liquid ejection head, it is necessary to apply a force more efficiently to the ink column being ejected from the ejection opening to produce greater flow changes in the ink column.